Postgresql is using seqscan when is should use indexes.

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We have two tables, dst_port_hour and dst_port_day, which should be
very similar, they both have about 50.000.000 rows. In both tables we
have an index for period_id.

We run postgresql 7.4.5 on a dedicated Debian server, with dual Intel
Xeon 3GHz and 4GB memory.

The problem is that on the dst_port_day table, postgresql is using
seqscan, and not the index when it should. Forcing the use of the index
by setting enable_seqscan to false, makes the query lighthening fast.
When using seqscan, the query takes several minutes. The planner
calculates the cost for Index scan to be much more than sequence scan.

Why is our query planner misbehaving?

Here are the exaplain analyze output with and without index-force:

SET enable_seqscan=false;

stager=> explain analyze SELECT cur.portnr FROM dst_port_day cur WHERE
cur.period_id='2779' GROUP BY cur.portnr ORDER BY SUM(cur.octets) DESC
LIMIT 5;

QUERY PLAN
- ------------------------------------------------------------------------
- ------------------------------------------------------------------------
- ---------------------------------
Limit (cost=2022664.62..2022664.63 rows=5 width=12) (actual
time=831.772..831.816 rows=5 loops=1)
-> Sort (cost=2022664.62..2022664.82 rows=80 width=12) (actual
time=831.761..831.774 rows=5 loops=1)
Sort Key: sum(octets)
-> HashAggregate (cost=2022661.89..2022662.09 rows=80
width=12) (actual time=587.036..663.991 rows=16396 loops=1)
-> Index Scan using dst_port_day_period_id_key on
dst_port_day cur (cost=0.00..2019931.14 rows=546150 width=12) (actual
time=0.038..303.801 rows=48072 loops=1)
Index Cond: (period_id = 2779)
Total runtime: 836.362 ms
(7 rows)

SET enable_seqscan=true;

stager=> explain analyze SELECT cur.portnr FROM dst_port_day cur
WHERE cur.period_id='2779' GROUP BY cur.portnr ORDER BY
SUM(cur.octets) DESC LIMIT 5;

QUERY PLAN
- ------------------------------------------------------------------------
- ------------------------------------------------------------------------
- ------
Limit (cost=1209426.88..1209426.89 rows=5 width=12) (actual
time=299053.006..299053.053 rows=5 loops=1)
-> Sort (cost=1209426.88..1209427.08 rows=80 width=12) (actual
time=299052.995..299053.008 rows=5 loops=1)
Sort Key: sum(octets)
-> HashAggregate (cost=1209424.15..1209424.35 rows=80
width=12) (actual time=298803.273..298881.020 rows=16396 loops=1)
-> Seq Scan on dst_port_day cur (cost=0.00..1206693.40
rows=546150 width=12) (actual time=298299.508..298526.544 rows=48072
loops=1)
Filter: (period_id = 2779)
Total runtime: 299057.643 ms
(7 rows)

- --
Andreas Åkre Solberg, UNINETT AS Testnett
Contact info and Public PGP Key available on:
http://andreas.solweb.no/?account=Work

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